1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus for reconstructing a magnetic resonance image on the basis of magnetic resonance signals from nucleuses excited in a subject with a radiofrequency magnetic field.
2. Description of the Related Art
A gradient unit for producing gradient magnetic fields is one of the most important components in a magnetic resonance imaging apparatus.
The gradient unit has three coil sets and three amplifiers to allow formation of three independent gradient magnetic fields in three orthogonal axes (X, Y, and Z). An X-axis gradient magnetic field GX, a Y-axis gradient magnetic field GY, and a Z-axis gradient magnetic field GZ are used, in the form of independent magnetic fields or a synthetic magnetic field, as a readout gradient magnetic field (GR) for spatially encoding the frequency of a magnetic resonance signal, a phase encoding gradient magnetic field (GE) for spatially encoding the phase of the magnetic resonance signal, and a slice selection gradient magnetic field (GS) for selecting the imaging volume of a subject.
Gradient magnetic coils of each axis generally form pairs of coils arranged to form a magnetic field distribution such that the magnetic field strength of a gap (imaging region) between each pair of coils linearly changes. There is known a so-called active shielded gradient coil unit in which a magnetic shield coil for generating a magnetic field in a direction opposite to that of the magnetic field generated by a primary coil for gradient field generation is arranged outside the primary coil in order to suppress generation of an eddy current in a peripheral conductor upon leakage of this gradient magnetic field to the exterior. FIG. 1 shows a circuit for generating a gradient magnetic field of one axis. This circuit comprises main coils 1 to 4 for generating a gradient magnetic field, shield coils 5 to 8 for magnetically shielding, from the exterior, the magnetic field generated by the primary coils 1 to 4, and amplifiers for supplying currents to these coils. A very high-speed imaging method represented by an echo planar imaging method (EPI method) has recently been put into practice. This very high-speed imaging method can acquire, on the order of several tens of msec, a plurality of echoes required for reconstructing one magnetic resonance image and has a high industrial applicability.
A so-called standard spin echo method can raise a gradient magnetic field having a magnetic field strength of 10 mT/m in 1 msec. However, according to the EPI method, a gradient magnetic field having a strength of 30 mT/m must be raised in 0.1 msec and alternated at high speed. An active shielded gradient coil unit is an indispensable unit to prevent an eddy current from being generated from the peripheral conductor by this high-speed alternating magnetic field.
It is difficult to drive the serially connected primary coils 1 to 4 and the serially connected shield coils 5 to 8 by one amplifier 10 to raise, within a short period of time, the magnetic fields having a relatively high strength which satisfies the echo planar method. For example, an amplifier having a high output of 4 kV or more is required.
Employment of an amplifier having a higher output requires improvements such as prevention of magnetic coupling of surrounding metals and surrounding coils (a static magnetic field coil, static magnetic field correcting coil and the RF coil) with respect to the gradient coils.
The space between the primary coils and the shield coils can be increased to reduce the impedance of the gradient coil, thereby moderating the required specifications of the amplifier. This, however, is not preferable because a magnet gantry becomes bulky. The above problem of magnetic coupling can be solved by increasing the space between the gradient coils and the static magnetic field coil and the space between the gradient coils and the RF coil. However, the magnet gantry becomes undesirably bulky.